Radiant Exchange
Heat Transfer at the Speed of Light (3 x 1010 cm/sec)
No medium required - can occur in vacuum
Not dependent on air temperature
Net transfer - Stefan-Boltzmann Law
Radiant Heat
Transfer
=
SB Constant x
Emiss.
Surf. 1
x
Emiss.
Surf. 2
x
(T14 - T24)
Significance of this transfer
Man - shorts - sitting quietly
~ 50 - 70% heat loss (30 W/m2) - via radiant exchange
Animal - bright sun - solar radiation (intercepted) =
much larger than MR
Total radiant power - received outside earth’s atmosphere
-on a plane - right angle to sun’s rays = 1360 W/m2
Atmosphere scatters light
Blue (shorter wavelength) more than red (longer wavelength)
>> blue sky
Sun - orange or red because blues & violets have been
scattered out + at sunset & sunrise - greater amount
atmosphere for light to pass through.
UV radiation diminished by:
1. Ozone absorption - stratosphere
2. scattering
Solar radiation - received by earth’s surface dependent on:
1. Sun’s elevation above horizon
2. Light scattering by atmosphere (including effects water droplets & ice particles - clouds
3. Absorbance - atmospheric gases (water vapor, CO2,
O3, etc....) - absorbs infrared radiation
Infrared radiation (sun) - almost entirely absorbed by
atmosphere
Visible & near-infrared (sun) pass through >> earth’s surface
- then trapped - reradiated as infrared from surface - but
cannot entirely leave
This = GREENHOUSE EFFECT by atmosphere >>
moderating effect on daily temperature swings of earth’s
surface.
Clear, dry atmosphere - night - rapid radiant cooling
Clear sky - night - serves as radiant heat sink
Low-temperature infrared radiation does not penetrate
water or tissues with water.
+ There is no effect on heat transfer within body
Color affects visible radiation absorption
Black absorbs more radiation - visible spectrum
White reflects more radiation visible spectrum
1/2 solar radiation reaching earth - in visible region
Would expect animals with dark coats or skin - to have
heat stress problems.
+ animals with light coats or skin to have few heatrelated problems.
NOT ALWAYS TRUE -
polar animals
Fur or plumage coats - absorption site = coat surface
Smooth or even surface exposed to solar radiation heat absorbed dependent on color.
Irregular coat - light color - beam reflected into
coat and absorbed near skin.
Dark color >> little reflectance - less penetration
Combine this with the effect of windspeed.
Temperature of superficial layers of insulation much
higher for dark plumage.
BUT - high wind speeds - heat absorbed - dark plumage much less - due - dissipation via convection.
Light plumage - less effect - wind speed - due to greater
penetration.
Coat density - important Sheep example
Awassi sheep - loose coat Deep penetration >> high skin temperature
Also - affected by wind speed
Merino sheep - dense coat little penetration
Skin temperature not as high
BUT - fleece temperature - very high
Large infrared heat loss
Large reduction - heat flow with increased fleece length
Ogaden sheep (Persian) - smooth white coats
Decreased heat load due to high reflectance of
solar radiation.
EMISSIVITY
Measurement of an objects ability to emit radiation at a
given temperature
Blackbody Emissivity = 1.0
Also an ideal absorber
•
Emissivity + Reflectivity + Transmittance = 1.0
Reflectivity = measurement of an object's ability to reflect
radiation
Transmittance = measurement of an object's ability to
pass or transmit radiation
• Ideal surface for infrared measurements is a perfect
radiator with an emissivity = 1
Most objects are not perfect radiators
Many instruments - compensate for different emissivities
Higher emissivity >> better chance getting accurate
Low emissivity objects = polished, shiny surfaces
• Most organic substances have emissivity = 0.95
Transmission - not an important consideration - except
in case of plastics and glass
BLACK GLOBE THERMOMETER
1) Practical / Inexpensive means - isolating mean radiant
temperature from other factors in - thermal
Environment
2) Indication of combined effects of radiant energy, air
temperature, and air velocity
______________________________________________
MEAN RADIANT TEMPERATURE
Temperature of a uniform "black" enclosure in which an
object would exchange same amount of energy as in
actual environment.
MRT = 100 {[Tg / 100]4 + 1.028 x sq. root [V(tg - ta)]}.25-460
Tg = tg + 460
V = air velocity (fpm)
tg = globe temperature (°F)
ta = air temperature (°F)
RADIANT HEAT LOAD
Total radiation received by an object from all surroundings
RHL = S x Ts4
Ts = MRT + 460
S = Stefan-Boltsman Constant = 0.173 x 10-8